1. Field of the Invention
The present invention relates to an antenna switching device or an antenna switching duplexer for use in a radio communication apparatus such as a digital portable radio telephone performing radio communication by selectively connecting an antenna to a transmitter or a receiver, i.e., using a TDMA (time division multiple access) system.
2. Description of the Related Art
In the past, a single-pole double throw switch (hereinafter, referred to as an SPDT switch) has been used for selectively connecting an antenna to a transmitter or a receiver in mobile communication equipment. An exemplary conventional SPDT switch will be described with reference to FIG. 7.
FIG. 7 shows a circuit configuration of a conventional SPDT switch. The SPDT switch includes a first signal terminal 601, a second signal terminal 602, a third signal terminal 603, a control terminal 604, PIN (positive-intrinsic-negative) diodes 605 and 606, a strip line 607 having a 1/4 wavelength of a signal frequency, a resistor 608, an inductor 609, and capacitors 610, 611, 612, and 613. The resistor 608, the inductor 609, and the capacitor 611 constitute a control circuit. One end of the capacitor 611 is connected to one end of the inductor 609, and the other end of the capacitor 611 is grounded. The capacitors 610, 612, and 613 function as direct-current blocking capacitors.
This conventional SPDT switch is used as a transmit-receive switch. The operation of the SPDT switch as a transmit-receive switch will be described. It is noted that in this case, the first, second, and third signal terminals 601, 602, and 603 work as a transmitting terminal, an antenna terminal, and a receiving terminal, respectively.
When a signal is transmitted from the antenna terminal, a positive voltage is applied to the control terminal 604. A control current flows through the control terminal 604, the resistor 608, the inductor 609, the PIN diode 605, the strip line 607, and the PIN diode 606 in this order. Associated with the flow of the control current, the impedance between an anode and a cathode of the PIN diodes 605 and 606 becomes low. The low impedance state between the anode and the cathode of the PIN diode 606 renders an electrical potential at one end of the strip line 607 connected to the anode of the PIN diode 606 at almost a ground level. This causes the impedance at the other end of the strip line 607 with respect to the antenna terminal 602 to be remarkably increased. As a result, a transmitting signal input from the transmitting terminal 601 is outputted from the antenna terminal 602 but is not outputted from the receiving terminal 603.
When a signal is received from the antenna terminal, no voltage is applied to the control terminal 604. Since no control current flows, the impedance between the anode and the cathode of the respective PIN diodes 605 and 606 becomes high. As a result, a receiving signal input from the antenna terminal 602 is outputted from the receiving terminal 503 but is not outputted from the transmitting terminal 601.
As described above, an antenna is selectively connected to a transmitter or a receiver by using the conventional SPDT switch.
However, in the case where the above-mentioned SPDT switch is applied to a diversity system capable of providing superior receiving characteristics in mobile radio communication, there arise problems. That is, since the diversity system utilizes two antennas, two SPDT switches are required for the system. In this case, the number of components included in a communication apparatus is increased, making it difficult to realize a miniaturized apparatus and rendering the loss of signal in the apparatus large.